CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 49 



the other. As Professor Huxley has graphically remarked, "regarded as 

 machines for doing certain kinds of work, animals differ from one another 

 in the extent to which this work is subdivided. Each subordinate group 

 of actions or functions is allotted to a particular portion of the body, 

 which thus becomes the organ of those functions ; and the extent to 

 which this division of physiological labour is carried differs in degree 

 within the limits of each common plan, and is the chief cause of the 

 diversity in the working out of the common plan of a group exhibited 

 by its members. Moreover, there are certain types which never 

 attain the same degree of physiological perfection as others do." 

 These words indicate clearly enough that the high or low character 

 of any animal in a type, depends chiefly upon the complexity of 

 the functions, and necessarily of the organs, whereby life is main- 

 tained. As the household whereof the labour is performed by a maid- 

 of-all-work, is functionally less complex than that whose work is per- 

 formed by a retinue of servants, each discharging a special duty, so 

 in the animal world, the rank of any one of its members or of its 

 groups can only be determined by the complexity of body, and 

 by the corresponding degree of intricacy with which the functions 

 of the body are performed. Furthermore, it is the development of 

 this complexity, or the reverse, from the common plan, which, as 

 Huxley has so well expressed it, is the actual cause of the variations 

 we see in each type. The frog is not higher than the fish because 

 of its type since both exhibit the same fundamental plan ; but 

 because the frog's functions are more specialised there is a more 

 minute physiological " division of labour " in the frog, and there exists 

 a more complex staff of organs (developed from the common plan 

 of fish and frog) to discharge the increased work. Of the differences 

 between a frog and a bird, and between both and a man, precisely 

 the same remark may be made. The higher or more complex life 

 involves and demands from the common type, the more complex 

 frame. 



To quote Professor Huxley's words once more, "a mill with ten pairs 

 of mill-stones need not be a more complicated machine than a mill 

 with one pair ; but if a mill have two pairs of mill- stones, one for 

 coarse and one for fine grinding, so arranged that the substance 

 ground passes from one to the other, then it is a more complicated 

 machine a machine of higher order than that with ten pairs of 

 similar grindstones. In other words, it is not mere multiplication of 

 organs which constitutes physiological differentiation ; but the multi- 

 plication of different organs for different functions in the first place, 

 and the degree in which they are co-ordinated, so as to work to a 

 common end, in the second place. Thus a lobster is a higher 

 animal, from a physiological point of view, than a Cyclops (or water- 

 flea), not because it has more distinguishable organs, but because 



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